The present invention relates to a liquid-crystalline dielectric having a particularly steep characteristic curve.
The properties whereby nematic or nematic-cholesteric liquid-crystalline materials significantly vary their optical properties, such as light absorption, light scattering, birefringence, reflectivity or color, under the influence of electric fields are increasingly utilized for electro-optical display elements. The functioning of display elements of this type is based, for example, on the phenomena of dynamic scattering, the deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the cholesteric-nematic phase transition.
For the industrial application of these effects in electronic components, liquid-crystalline dielectrics are required which must fulfill a large number of requirements. Chemical resistance to moisture, air and physical influences, such as heat, radiation in the infrared, visible and ultraviolet ranges, and continuous and alternating electric fields, is of particular importance. Industrially usable liquid-crystalline dielectrics are also required to have a liquid-crystalline mesophase in the temperature range from at least +10.degree. C. to +50.degree. C., preferably from 0.degree. C. to 60.degree. C., and the lowest possible viscosity at room temperature, which preferably should not exceed 70.times.10.sup.- Pa.s. Finally, they must not have any characteristic absorption in the range of visible light, i.e., they must be colorless.
None of the hitherto known classes of compounds having a liquid-crystalline mesophase includes an individual compound which forms a liquid-crystalline nematic mesophase within the required temperature range from 0.degree. C. to 60.degree. C. As a rule, therefore, mixtures of two or more compounds are prepared in order to obtain substances which can be used as liquid-crystalline dielectrics. For this purpose, at least one compound having a low melting point and clear point is usually mixed with another compound having a markedly higher melting point and clear point. This normally provides a mixture whose melting point is below that of the lower-melting component, while the clear point is between the clear points of the components. It is, however, not easy to prepare optimum dielectrics in this way, since the components having the high melting points and clear points frequently also impart a high viscosity to the mixtures. As a result, the switching times of the electro-optical display elements produced with these mixtures are extended in an undesirable manner.
Moreover, for all the types of display elements mentioned above, it is important that the contrast/voltage curve be as steep as possible in the region of the threshold voltage; that is, the display should be immediately activated up to the fullest possible contrast when the threshold voltage is slightly exceeded. Additionally, the threshold voltage itself should have only the lowest possible temperature dependence, so that the threshold voltages required for activating the display, especially at low temperatures, are not substantially higher than those required, for example, at room temperature.
Particularly valuable liquid-crystalline base materials which are already very widely used in liquid-crystalline dielectrics and which meet the above mentioned stability requirements and are colorless, include, in particular, the phenylcyclohexane derivatives or biphenylcyclohexane derivatives of formula (II) ##STR3## the alkylcyano-biphenyls or -terphenyls of the formula (III) ##STR4## the alkoxycyano-biphenyls or -terphenyls of the formula (IV) ##STR5## the cyclohexylcyclohexanes of the formula (V) ##STR6## and the phenylpyrimidines of the formula (VI) ##STR7## in which formulae (II) to (VI) n is 1 or 2, and Z is R', OR' or CN, R and R' being the same or different, and each being alkyl of 1-7 C atoms.
A large number of liquid-crystalline dielectrics based on these compounds is already commercially available. There is, however, still a great demand for liquid-crystalline dielectrics having the steepest possible characteristic curve (contrast/voltage curve), in particular for matrix display elements. In such matrix display elements, each "display point" of an electrode grid can be selectively triggered by applying a voltage to a first electrode layer consisting of a multiplicity of horizontal rows of conductors, and to another electrode layer consisting of a multiplicity of vertical rows of conductors. A disadvantage of these matrix display elements is the partial activation of display points in the immediate vicinity of a triggered display point, which partial activation is called "crosstalk" and reduces the contrast of the display in an undesirable manner. The steeper the characteristic curve of the liquid-crystalline dielectric used in a display element of this type, the less such cross-talk is observed.
As a rule, the steepness of the characteristic curve of a liquid-crystalline dielectric is expressed as the ratio of the control voltages V which must be applied to a given liquid crystal display element in order to obtain 90% of the maximum contrast (V.sub.90) and 10% of the maximum contrast (V.sub.10). The steepness of the characteristic curve is the greater, the smaller the ratio EQU .gamma.=(V.sub.90 /V.sub.10)